Wednesday, November 17, 2010

Effects of sleep on Remembering to Remember

Prospective memory is a class of memory that is unique in that it involves the future rather than the present or the past. Various examples of prospective memory include remembering to buy a pet monkey, or remembering to break up with your girlfriend or remembering to do that blasted thesis that you have been putting off for the umpteenth time. Hence, some researchers have called it the act of "remembering to remember" (Winograd, 1988). Although prospective memory is quite important for daily functioning and goal fulfilment, research on it has been quite sparse in comparison to other forms of memory. One big question remains: how do we remember to remember? Do we devote some resources to maintain this intention? Or is it maintained without anything being devoted to it?

You know you want it.

One theory of prospective memory suggests that prospective memory can be both effortful and automatic. One way in which prospective memory can be automatic is when cues in our environment alert us to something we have to do. Kvavilashvili and Fisher (2007) has found that individuals often encounter cues related to a goal (eg, seeing a monkey in the magazine, receiving a text message from the girlfriend) and these cues spontaneously remind participants of their goal. Another way in which prospective memory is maintained is through conscious retrieval. Other researchers have also found that prospective memory surprisingly is better after 15 minutes than 3 minutes, one interpretation being that participants consciously retrieve and rehearse the goal prior to and up to the point when they are going to execute it (Hicks, Marsh & Cook, 2005).

One question that arises from these findings is that, what happens when we sleep on something? During sleep it appears that both automatic and effortful mechanisms are absent; we don't consciously retrieve and rehearse goals, and we aren't exposed to cues from the external environment. In such a case we might expect that prospective memory would weaken. However, given that a huge amount of literature has also shown that sleep improves memory, we might expect that sleep would improve prospective memory. These contradictory hypotheses form the basis for Scullin and McDaniel's study, "Remembering to execute a Goal: Sleep on it!" (2010)

The researchers had participants do a series of tasks (too lengthy to cover), while remembering to do a particular action at a particular time ("press the Q key when you see the word table or horse"). In one set of participants, this target came after a short delay (20 minutes) while in the long delay condition, participants were told to come back after 12 hours for another series of experiments and the target came there. The other crucial manipulation was time of day, the experiment for one set of participants was started in the morning, and the other set at night. The study was between groups, so participants were spread out across 4 conditions.

1. Short Morning Delay

2. Short Evening Delay

3. Wake Delay (from morning to night, during which time the participant was awake)

4. Sleep Delay (from night to morning, during which time the participant was sleeping)

What the researchers found was that sleeping improved the ability to remember to do something, as compared to being awake. In some of the tests, the performance was almost as good as if one was tested after 20 minutes. One important possible confound in this experiment was that the sleep and awake conditions also differed not just in amount of sleep but also when the experiment was conducted. The authors eliminated this confound by comparing performance for prospective memory during the day and night after a short delay and found that time of day had little effect.

So the results are inconsistent with the idea that sleep might undermine prospective memory by preventing conscious retrieval or exposure to environmental stimuli. It seems like prospective memory might not rely heavily on these 2 processes, as suggested by some proponents of the theory above. The implication being that if you need to remember to do something, you might as well sleep on it.

But how does sleep improve prospective memory? The authors suggest that sleep increases associative binding and that it amplifies weak goals or links. In this case, it seems like prospective memory might not be very much different from other types of memory. However the authors find that sleep improves prospective memory for only certain types of tasks, thus raising new avenues for research into different kinds of prospective memory and how they are affected by different variables.

One problem which I found in this paper was that the participants were engaging in quite an unnatural prospective memory task, in which case environmental cues present during the awake phase would not help the participant in remembering to do the task. On the other hand, in the real world, such environmental cues would be present and abundant (like seeing your thesis supervisor). So in that sense, the authors can't make the claim that sleeping improves prospective memory more than staying awake and being exposed to constant pictures of monkeys.

Another interesting idea that this paper raises is the implication of sleep on prospective memory in old people. It has been found that prospective memory is one memory that is especially impaired and gets worse with old age (they keep forgetting to get the groceries for instance). Old people also get less sleep, especially sleep of the slow wave kind. Coincidence? :)